Grid-connected power generation systems typically include two major parts: power generators that produce the power and converters that receive, condition, and inject the power into the power distribution grid. Power generators include, for example, photovoltaic (PV) cells, fuel cells, and wind turbines.
To increase the overall efficiency of power generators under different circumstances, such as partial shadowing of PV cells, or mismatches between PV cells or wind turbines, independent control and power extraction is required for each power generator. This requires using a separate converter for each power generator. This may be referred to as micro-inverter technology.
In single-phase grid-connected power generation systems, the instantaneous output power oscillates at twice the grid frequency. In systems where the power generators are PV cells, the input power generation is DC and thus oscillation of the instantaneous power at the converter output, if reflected in the input, causes the input operating point to deviate from DC. If there is any power oscillation on the PV cell side, maximum power is only achievable at the peak of oscillation, which translates into less average power extraction than the available maximum power [1,2]. This is a power loss that reduces the efficiency of the PV cell system. Therefore, power pulsation is a key problem in such systems and the PV cell converter should decouple the output power pulsation from the input DC power generation to maximize the efficiency.
As noted above, if there is no power decoupling in a single-phase inverter, the power generation at the PV cell terminal will contain oscillations that result in a deviation from the optimum point. Energy storage in the circuit may supply oscillatory power and reduce power pulsation at the PV cell terminal. The decoupling problem is normally resolved by using large electrolytic capacitors (e.g., in the range of milli-Farads) to minimize the effect of the output power pulsation on the input operating point. However, this is highly undesirable because it decreases the life-time and increases the volume, weight, and cost of the inverter.
Depending on the topology, different locations of the energy storage are possible. For example, for a single-stage topology energy storage may be implemented at the PV cell terminals. For multi-stage topologies, when a voltage source inverter is employed at the output, the power decoupling capacitor may be placed at the input terminals and/or at the DC bus (e.g., between DC-DC converter and DC-AC inverter stages). It is beneficial to have most of the decoupling capacitance on the DC bus because the voltage level is higher and the same amount of energy storage can be achieved with a smaller capacitor.
The generation of a high DC voltage is not efficient and it poses an excessive voltage stress on the inverter and on the output of the first stage. Moreover, the high voltage on the bus enlarges high frequency ripples on the output current, which requires large passive filters for compensation. Further, in such an approach relatively large electrolytic capacitors at the input are still needed to remove any oscillations at the PV cell input.
Use of a voltage source inverter at the output requires a bulky inductor for connection to the grid. To avoid this, a micro-inverter may use an unfolding power circuit in the last stage. However, with this approach, a large electrolytic capacitor bank is still required at the PV cell terminals because the voltage level is very low and the amount of capacitance required becomes large. In general, topologies that use a transformer as an energy buffer employ such a configuration for power decoupling [1,2,5].
To reduce the amount of input capacitance required, a multistage approach may be used as in [3] which processes the full output power. However, this reduces the efficiency and increases the size and weight of the converter. Moreover, since the DC bus voltage is very high the stress on the switches of the converter stages is very high and also the filter that shapes the current becomes relatively large.
In other approaches [5-10], an auxiliary power circuit is introduced that absorbs power and provides energy when needed. As a result a large electrolytic capacitor is not required. The auxiliary power circuit usually operates at high voltage to reduce the energy storage component. Such approaches generally have low efficiency and have high number of power processing stages.